1. Field of the Invention
The invention concerns a cathode.
2. Description of the Prior Art
Cathodes that have a spiral emitter (spiral-wound filament) or surface emitter are used in x-ray tubes, for example. A cathode with a spiral emitter is known from DE 199 55 845 A1, for example. Cathodes that have surface emitters are described in DE 27 27 907 C2 and in DE 199 14 739 C1, for example.
In operation of the x-ray tube, heating voltage is applied to the spiral emitter or to the surface emitter, causing electrons to be emitted that are accelerated in the direction of an anode. X-ray radiation is generated in the surface of the anode upon the electrons striking the anode.
The high temperature of the spiral-wound filament produces a vaporization of the material (tungsten), and a slow thinning of the spiral filament results from this that ultimately leads to a fracture of the spiral filament. This effect is generally known in filament lamps.
A reduction of the wear, and an associated increase of the lifespan can be achieved only by a reduction of the operating temperature of the spiral emitter, but this leads to an unwanted reduction of the electron emission. In order to prevent a reduction of the electron emission due to reduced operating temperature of the spiral emitter, a particularly simple measure that lends itself to the situation is to make the radiating surface for the electron emission comparably large without having to use significantly higher heating currents. Given a suitable design, such a surface emitter has a distinctly larger radiating surface usable for emission relative to the volume to be heated and in comparison to a spiral emitter.
In spite of the greater lifespan of a surface emitter, as before spiral transmitters are nevertheless used since, among other things, surface emitters can be most difficult to block via electrical fields due to their larger radiating surface (emission surface). This blocking by application of a negative voltage at the cathode head is necessary in many applications, in particular given application with pulsed x-ray radiation. Particularly the more central regions of large-area surface emitters are geometrically further removed from the electron accumulations generating the barrier field at the cathode head, and thus can be blocked only by higher electron concentrations or, respectively, higher field strengths. Higher field strengths in turn entail greater minimum distances to be maintained (to avoid flash-overs) as well as additional construction expenditure.